Aromatic polycarboxylic acids have been produced through the oxidation of the corresponding alkyl group with molecular oxygen. Examples of such acids are pure terephthalic acid (PTA), isophthalic acid (IPA), trimellitic acid (TMA), 2,6-naphthalene dicarboxylic acid (2,6-NDA), 2,7-naphthalene dicarboxylic acid (2,7-NDA), and others. Since PTA is the most typical process, it will be used for illustrations in the invention. However, the purification and production methods of the instant invention are applicable for all aromatic polycarboxylic acids and derivatives thereof.
A predominant process for making PTA consists of the following steps to prepare crude terephthalic acid (CTA).
1) Oxidization: The reaction of p-xylene (PX) with air is carried out in a liquid phase at 150-230.degree. C. and 150-425 psia using cobalt-manganese-bromine as catalysts and acetic acid as solvent. PA1 2) Crystallization: The effluent from the reactor is crystallized through 3 to 5 large crystallizers at a reduced pressure and temperature to precipitate terephthalic acid from mother liquor. PA1 3) Filtration: The crude acid is then separated from mother liquor by centrifugation/filtration. The mother liquor, with or without treatment, is recycled to the oxidation step. PA1 4) Drying: The crude acid is dried by blowing inert gas, and the acetic acid carried by inert gas is then recovered by a scrubber. The dried crude terephthalic acid is pneumatically carried to a silo or storage bin that requires large nitrogen flow or an air separation plant for some PTA plants. PA1 5) Solvents and catalysts recovery: Solvent and catalysts are recovered by various processes.
CTA containing about 0.5% impurities is then purified by a hydrogenation process to produce polymer-grade PTA containing about 25 PPM of 4-carboxybenzaldehyde (4-CBA), 150 PPM of p-toluic acid, and about 0-50 PPM of benzoic acid. Similar to CTA, the purified PTA from hydrogenation unit goes through another set of process steps: crystallization; filtration; and drying as described above. Thus, to remove impurities from reactor effluent at about 0.5% to a purified product at about 0.025%, the predominate process uses the following expensive steps:
1) Requiring two sets of process steps for crystallization, centrifugation/filtration, drying, and pneumatically carrying equipment. PA0 2) Using expensive purification process by chemical reaction. Disregarding the higher capital cost of hydrogenation unit, high production cost is required because of operating under high temperature and pressure by using expensive noble metals as catalyst. PA0 3) Requiring long resident time for crystallization. CTA takes about 3-5, and PTA takes about 5, large crystallizers to recover product from mother liquor. In addition, due to highly corrosive bromine-acetic acid environment, some crystallizers may require using expensive corrosive-resistant material, such as titanium-lined equipment. PA0 4) Requiring drying and pneumatically carrying to make finished product. PA0 5) Meeting polymer-grade specification, but product still containing about 0.01% of impurities. PA0 1) Dissolving crude acid: The aromatic polycarboxylic acid forms salt with many base compounds, and the salt is soluble in a dissolving solvent such as water or alcohol at elevated temperature. PA0 2) Removing impurities: Some impurities can be easily separated by solution pretreatment, such as activated carbon for colorants. The impurities having close properties with the acid are separated in mother liquor by crystallizing with cooling for at least 30.degree. C. PA0 3) Recovering product: Hirowatari thermally decomposes the solution from pretreatment by heating or contacting steam with a concentrated solution in the presence of alkylene glycol. Iwane precipitates and washes the salt that is then converted to a purified product by thermally decomposing or by adding an acid-substitution solvent to substitute the product acid in the salt Iwane also recovers product by directly adding an acid-substitution solvent to the solution. PA0 1) Introducing base compound that contaminates the finished product. PA0 2) Requiring two sets of process steps for crystallization, centrifugation/filtration, drying, and pneumatically carrying equipment. PA0 3) Significant detectable impurities remaining in the purified product. PA0 4) Requiring long resident time for crystallization, and thus, several large crystallizers. PA0 5) Requiring drying and pneumatically carrying to make finished product. PA0 1) To provide a solvent extraction purification method to remove impurities from the crude acid to meet or exceed the specification of polymer-grade aromatic polycarboxylic acids. PA0 2) To remove the base compound from purified product so that it will be suitable for making polyester fibers, films, molding resin, or other applications. PA0 3) To provide a process for producing polymer-grade aromatic polycarboxylic acids that requires only one set of process steps to substantially reduce capital and production costs. PA0 4) To reduce the number of crystallizers required for crystallization, or eliminate them. PA0 5) To produce aromatic polycarboxylic acids that may be used directly for making polyesters without requiring drying and pneumatically carrying steps. PA0 1) In addition to CTA, the purification method may directly take the reactor effluent as feed without separating CTA. The effluent contains other material, such as catalysts (including catalyst promoters) and acetic acid, that can be separated from the product in the purification process. This allows reducing two sets of process steps in the predominant process to one set to significantly reduce the capital and production costs. PA0 2) The prior art has to consider factors such as viscosity, particle size, product recovery, and inclusion of impurity that can be circumvented by the proposed crystallization with direct-extraction from salt. In addition, the base-extraction solvent can be used to adjust the viscosity of slurry for separation and transportation. PA0 3) The purification method can remove more impurities than the hydrogenation unit in the predominant process. This allows the oxidation reactor to operate at more economical condition, such as a severity for lower hydrocarbon combustion and catalyst consumption, etc. PA0 4) The purification method does not require crystallizer for crystallization. PA0 5) The purification method may eliminate drying and pneumatically carrying steps by using evaporation with thermal decomposition by alkylene glycol. PA0 6) The purification method uses physical separation rather than chemical reaction to remove impurities, this requires less capital and production costs for purification. PA0 7) The purification method produces product purity significantly higher than the predominant process. This provides many potential advantages as described previously.
PTA, or derivative thereof, in high purity is required to be suitable for making polyester fibers, films, and molding resin. Terephthalic acid is difficult to be purified due to its low solubility in most solvents, high boiling temperature, and similarities in physical and chemical properties with impurities present.
An alternative is to remove impurities by solvent extraction. The solvent extraction approach is attractive because of lower costs. It can be the traced back to 1953 (U.S. Pat. No. 2,664,440), or even earlier. In early stage, solvents suggested are unstable, reactive with the product, toxic, or unable to purify CTA to desired level. Thereafter, Iwane (U.S. Pat. No. 5,344,969) and Hirowatari (U.S. Pat. No. 5,565,609) disclosed methods that use more stable solvents. The following summarizes these methods.
Both Iwane and Hirowatari use amine compound consisting of nitrogen as the only hetero atom, such as aliphatic, alicyclic, aromatic, or heterocyclic amines. Iwane uses an alcohol as the dissolving solvent for the purification of crude NDA from oxidation. Hirowatari uses water as the dissolving solvent to recover aromatic dicarboxylic acids from hydrolyzed polyester resins. In his approach, no purified salt is prepared because its impurities consist of only additives and colorants that can be simply removed by activated carbon. Thus, this method is suitable for purifying hydrolyzed resins containing already highly purified PTA with colorants or additives that are easy to be separated, but not for the crude aromatic dicarboxylic acids from oxidation containing impurities that are difficult to be separated.
For thermal decomposition, Iwane adds heat to the salt that may be dispersed in a paraffin, alkylbenzene, alkylnaphthalene, or alkylbiphenyl, and does not use steam for heating. The chosen solvents have high boiling temperature that will be presented in the finished product as another contaminant. Hirowatari heats the pretreated aqueous solution while refluxing to decompose the amine salt, or concentrates the solution by distillation before contacting with steam to decompose and remove the amine compound. Alkylene glycol is used to raise reflux temperature. The refluxing increases the content of base compound in the finished product, and the distillation has to evaporate more than 50% of water that requires significant energy.
Iwane claims improving 2,6-NDA purity from 97.2% to about 99.8%, and Hirowatari recovers a hydrolyzed resin to a 99.9% PTA. Iwane applies his method to crude NDA from reactor effluent at a purity level lower than CTA that is purified to a level only close to CTA. Although both approaches improves product purity, they are still off from the specification of polymer-grade PTA (&gt;99.98%).
The other approach is Lee (U.S. Pat. No. 5,767,311) that uses N-methyl pyrrolidone (NMP) to dissolve CTA between 140-190.degree. C. without using a dissolving solvent. The solution is cooled to 5-50.degree. C. for crystallization. Filtering and washing the precipitate make a PTA meeting polymer-grade specification without using means to recover product from salt. However, experiments using this method indicate that unconverted salts contaminate the finished product. The contamination may be from the failure to recognize the existence of salt formed by NMP and PTA in the process. Lee identifies the precipitation from solution as PTA, but it is actually a salt. The salt is converted to product during washing by some of his washing solvents, such as methanol. However, significant salts are unconverted because only washing is insufficient to convert all salts to product. The dissolution process and solvent recovery of the method are expensive. Compared with amine compound or morpholine, NMP is about 2-3 times more expensive and requires 3-5 times more to dissolve the crude acid. It also costs more to heat and recover the high boiling solvent. In addition, Lee incorrectly asserts that CTA can be dissolved in a nonaqueous morpholine solution. Its solubility is negligible disregarding solution temperature unless water is presented. Even if morpholine were able to dissolve CTA, the finished product is not a PTA but a salt, because methanol cannot convert morpholine salt to PTA. The difference in NMP and morpholine salts will be further discussed and taken advantaged by the present invention. The present invention uses new crystallization process and washing solvent to improve product quality of this method.
Disregarding the advantages there is no known commercial application of purification by solvent extraction. A major problem is from the fact that the residual base compound remaining in the finished product becomes a contaminant itself. All proposed organic base compounds contain nitrogen that causes color and other problems in making polyesters, and no prior art discusses the problem and teaches how to remove the base compound from the finished product.
Crystals always contain residual solvent by inclusion during crystallization. Using the known methods, it may contain more than 0.1% of residual base compound that is close to the impurity level of CTA. To be suitable for making polyesters, the residual base compound has to be removed to a few parts per million that is close to the impurity level of PTA. Therefore, the previously known prior art of solvent extraction removes impurities in the crude acid, but introduces residual base compound as contaminant in the product that makes it unsuitable for making polyesters.
The known methods do not attempt to remove residual base compound from the finished product. Iwane teaches using acid solvent, Hirowatari teaches using water to wash base compound from the filter finished product cake, and Lee teaches using NMP, p-xylene, acetone, methyl ethyl ketone, or methanol to wash the filter cake. An experiment using 100:1 ratio of water to wash and leach the cake for about 10 hours, produces a purified product still contains significant amount of base compound. This indicates that the base compound is difficult to be removed once it is included in product crystalline. This problem is either unknown for having not been addressed in open literatures, or known by those highly skilled in the art, but remained to be unsolved.
The prior art of solvent extraction purification methods either specifically or implicitly suggest replacing hydrogenation unit and uses CTA as feed except Lee. Lee incorrectly asserts that high percentage of CTA can be directly recovered from filtering reactor effluent without using crystallization or other means. Because most CTA remain in the mother liquor of reactor effluent, and it requires long residence time to precipitate CTA. Therefore, the predominated process uses 3-5 large crystallizers to recover CTA. The instant invention suggests using flashing and evaporation to reduce residence time.
Because the purification method itself needs another set of process steps for crystallization, filtration, drying, and pneumatically carrying, the prior art also requires two sets of process steps to produce PTA. Thus, using the known methods of solvent extraction for producing aromatic polycarboxylic acids suffers the following disadvantages:
Accordingly, several objects of the present invention are:
Further objects and advantages will become apparent from the disclosed solvent extraction method that unexpectedly and surprisingly removes impurities to undetectable level by the current standard HPLC measurement.